1. Field of the Invention
The present invention relates to eccentricity compensation in a disk drive servo system, and more particularly, to an eccentricity compensation apparatus of a disk drive servo system which is effectively adapted to a variation in reproduction speed of a disk by adjusting a gain and phase of control data estimated at a specific reproduction speed and stored in a feedforward look-up table, based on frequency response characteristics of an actuator.
2. Description of the Related Art
Eccentricity occurs when the center of a disk track deviates from an axis of a spindle about which a disk rotates. The eccentricity acts as periodic disturbance on a disk drive system, thereby significantly degrading track following performance thereof. In particular, as speed at which a disk is reproduced (e.g., disk reproduction speed) increases, eccentricity increasingly degrades the control performance of a disk drive system. Thus, for example, to accurately trace a track at high reproduction speed, eccentricity needs to be effectively compensated for. A number of techniques have been developed to compensate for eccentricity.
FIG. 1 shows an example of a conventional disk drive servo system compensating for eccentricity.
Referring now to FIG. 1, the conventional disk drive servo system includes an optical system 100, a feedback controller 110, a feedforward look-up table 120, and an actuator 130. The optical system 100 detects an error e(t) between the actual position of a head on a disk and a reference head position d(t), which corresponds to a displacement of the actuator 130, and outputs the error e(t) detected. The feedback controller 110 performs compensation control according to the error e(t) to output an actuator drive control value Ufb(t). The feedforward look-up table 120 outputs an actuator drive control value Uff(t) which is estimated and stored according to a phase of a spindle to compensate for eccentricity. The actuator 130 is moved to a position on the disk by a control value U(t) which is obtained by the actuator drive control values Ufb(t) and Uff(t), output from the feedback controller 110 and the feedforward look-up table 120, respectively.
The conventional disk drive servo system compensates for eccentricity by storing a compensation value for eccentricity compensation in the feedforward look-up table 120, and outputting a result of a summation of the compensation value and the control value output from the feedback controller 110, as the actuator drive control value. Typically, for example, eccentricity varies depending on a phase of a spindle.
Eccentricity present in the disk drive servo system has constant magnitude and varying frequency as the disk reproduction speed changes. The impact of eccentricity upon the system due to changes in the disk reproduction speed varies depending on the frequency response characteristics of the actuator 130. Thus, for example, to effectively compensate for eccentricity, data of the feedforward look-up table 120 needs to be updated each time the disk reproduction speed changes. Since eccentricity becomes significantly larger at high reproduction speeds, the feedback controller 110 does not appropriately perform a tracking control. In this case, data of the feedforward look-up table 120 is not updated, thereby making eccentricity compensation of the actuator 130 impossible.